WSB: In dwindling forest, caribou lay low to avoid biting flies

Julia John

As the timber industry cuts into the boreal habitat of threatened woodland caribou (Rangifer tarandus caribou), Canadian law shields the species from hunting, but the caribou have little protection against a small but pesky consequence of forest loss.

Researchers in eastern Canada found that woodland caribou, which are susceptible to parasitic horse flies during the summer, compensate by limiting their movement to attract fewer of the persistent and painful biters, which are prevalent where logging has removed mature trees.

Over the summers of 2011 and 2012, the scientists compared the activity of 30 radio-collared caribou in northern Ontario with the abundance of biting flies. They caught and counted 15 species of bloodsucking horse flies, black flies and mosquitoes that swarmed them daily in forest stands inhabited by the caribou.

When horse fly numbers are high, the caribou “stay immobile, lie down, wait until the numbers back off at the tail of each day and continue their behavior,” said David Beresford, corresponding author on the study published in the Wildlife Society Bulletin and a biology professor at Ontario’s Trent University. “They outwait the biting. They slow up. Horse flies are visual hunters. If you’re a moving target, you’re going to be like the pied piper walking through the woods collecting them. Caribou stay still to not attract new ones.”

This finding differs from previous studies on barren land caribou (Rangifer tarandus groenlandicus), he said, which were found to flee to windy or cool places in the open landscape to escape biting flies. But it aligns with research conducted in Africa in the 1970s, which demonstrated that mobile oxen suffered more horse flies than motionless ones.

“Recently harvested stands tend to produce a lot more horse flies than mature stands,” Beresford said. “Insect harassment is an element of habitat loss that’s overlooked. It is tied directly to harvesting. You should have harvesting that still can provide refuge stands. Old stand has much more variable habitat than new stand, in which everything is zeroed from harvest time, creating habitat where horse flies visually can find targets.”

He plans to return to the woods this summer to evaluate the difference in the number of biting flies that unmoving and moving animals experience.

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Julia John is a science writer at The Wildlife Society. Contact her at jjohn@wildlife.org with any questions or comments about her article.